Sizing load bank conductors

[JES2727]

I'm connecting a 400KW load bank to a 650KW generator, 480V. That calculates to about 481 amps per phase fully loaded. The steps in the load bank are fused at : 2 @ 80 amps, 3 @150 amps. That's 610 amps. It seems that the load bank is fused higher than it's rated kw. Does anyone know if there's a reason for this, or if my calculations are wrong, or if something else is amiss? How shall I size the conductors?
Thank you.

 

[mayanees]

I would size the cable and the OCPD at 400kW*1.25/1.732/480 = 600 amps.
If the load test is expected to exceed 3 hours, then use a 100% rated breaker.
John M

 

[MIEngineer]

I would size the cable and the OCPD at 400kW*1.25/1.732/480 = 600 amps.
If the load test is expected to exceed 3 hours, then use a 100% rated breaker.
John M

For my own education... Why multiply by 1.25? Is that to convert the kW to kVA assuming an 80% PF? Would a load bank have a PF < 100%?

Thanks,

 

[mayanees]

For my own education... Why multiply by 1.25? Is that to convert the kW to kVA assuming an 80% PF? Would a load bank have a PF < 100%?

Thanks,

MIE,

I looked at the load bank as a continuous load, and therefore gave it a 125% multiplier - like you'd do for a motor.
The load bank has a 100% power factor for the purpose of calculations. There's probably a 10 HP blower motor on it with an 85% pf, but the bulk of the load is resistive.

John M

 

[JES2727]

For my own education... Why multiply by 1.25? Is that to convert the kW to kVA assuming an 80% PF? Would a load bank have a PF < 100%?

Thanks,

Well here's the word from the manufacturer : The fuses are sized at 125% to compensate for voltage spikes and high ambient temperatures. The load bank in question will pull 480 amps when fully loaded, but if they were to fuse it at 480 amps they would be changing a lot of fuses.
I will size my wire for 600 amps.

 

[brian john]

Depends on the length of the load test, duration of the step loads typically something like 10, 25, 50, 80 and 100%. We seldom run them at 100% fro a long period of time if this was my test I'd run two 4/0's. Assuming a 4 hour test at the noted percentages, we watch (IR) the conductors during the test.

 

[JES2727]

Depends on the length of the load test, duration of the step loads typically something like 10, 25, 50, 80 and 100%. We seldom run them at 100% fro a long period of time if this was my test I'd run two 4/0's. Assuming a 4 hour test at the noted percentages, we watch (IR) the conductors during the test.

Thank you.
This is a permanent installation, wet stacking has been an issue with this gen. It is oversized for the facility it serves. The gen is exercised weekly for one hour, but the load bank will also be utilized when the gen assumes the building load during power outages. I expect the load bank to be fully loaded very rarely, if ever. As I see it, however, I still need to size the conductors for the full load. I intend to install two sets of 350's....